correctly identify already linked packages when global prefix is a symlink

[nlf]

the previous behavior caused any linked package to be identified as missing when the global prefix is a symlink.

this in turn caused arborist to attempt to install it even though it was already there, which in the case of a linked package could cause the locally linked copy to be overwritten by a published version from the registry

References

Fixes #2104

[npm-deploy-user]

found 1 benchmarks with statistically significant performance improvements

• app-large: cache-only

found 1 benchmarks with statistically significant performance regressions

• app-medium: no-clean:audit

timing results

app-large	clean	lock-only	cache-only	cache-only legacy-peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@7	50.526 ±0.57	31.072 ±0.34	33.082 ±3.08	28.954 ±1.40	2.383 ±0.01	2.362 ±0.02	1.706 ±0.00	12.075 ±0.08	1.678 ±0.01	3.607 ±0.03
#2486	49.728 ±0.49	31.056 ±0.17	29.665 ±0.31	27.473 ±1.11	2.398 ±0.00	2.410 ±0.04	1.685 ±0.00	12.154 ±0.01	1.707 ±0.01	3.847 ±0.03

app-medium	clean	lock-only	cache-only	cache-only legacy-peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@7	37.827 ±0.46	21.967 ±0.15	20.671 ±1.08	18.983 ±0.04	2.344 ±0.00	2.285 ±0.00	1.612 ±0.01	7.835 ±0.01	1.581 ±0.00	2.843 ±0.02
#2486	35.135 ±0.63	21.875 ±0.06	20.677 ±0.06	20.947 ±0.10	2.251 ±0.02	2.262 ±0.01	1.614 ±0.04	7.806 ±0.01	1.607 ±0.02	3.472 ±0.00